Patent application title: Metal composite for fuel cell and fuel cell bipolar plate using same, and fabrication method for same

Abstract:

A metal composite for fuel cells according to the present invention, which
includes: a core of a metal; cladded layers of a corrosion resistant
metal covering both surfaces of the core; and a through-hole formed
through the core and cladded layers. The through-hole has, on a hole wall
of the core region of the through-hole, a concave portion which is
recessed relative to hole walls of the cladded layer regions of the
through-hole.

Claims:

1. A metal composite for fuel cells, comprising:a core of a metal;cladded
layers of a corrosion resistant metal covering both surfaces of the core;
anda through-hole formed through the core and cladded layers, the
through-hole having, on a hole wall of the core region of the
through-hole, a concave portion which is recessed relative to hole walls
of the cladded layer regions of the through-hole.

2. The metal composite according to claim 1, wherein:projected hole wall
portion of each cladded layer region of the through-hole is bent
inwardly.

3. The metal composite according to claim 1, wherein:the concave portion
is filled with a dissolution-inhibiting material for preventing
dissolution of the core.

4. A fuel cell bipolar plate fabricated from the metal composite according
to claim 1.

5. A fabrication method for a metal composite for fuel cells, comprising
steps of:covering both surfaces of a core of a metal with a cladded layer
of a corrosion resistant metal;forming a through-hole through the core
and cladded layers; andetching a hole wall of the core region of the
through-hole to form a concave portion which is recessed relative to hole
walls of the cladded layer regions of the through-hole.

6. The fabrication method according to claim 5, further comprising step
of:after the step of formation of the concave portion, forcing a punch
against and inwardly bending projected hole wall portion of each cladded
layer region of the through-hole which is projected relative to the hole
wall of the core region of the through-hole.

7. The fabrication method according to claim 5, further comprising steps
of:anodizing a surface of the concave portion; andapplying a boehmite
treatment to the anodized surface.

8. The fabrication method according to claim 5, further comprising step
of:filling a dissolution-inhibiting material to the concave portion.

Description:

CLAIM OF PRIORITY

[0001]The present application claims priority from Japanese patent
application serial no. 2007-243963 filed on Sep. 20, 2007, the content of
which is hereby incorporated by reference into this application.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to metal composites for fuel cells,
which have a core cladded with a corrosion resistant metal and have
through-holes formed therethrough, and fuel cell bipolar plates
fabricated from such a metal composite. The present invention also
relates to fabrication methods for the metal composites.

[0004]2. Description of Related Art

[0005]Conventionally, metal composites formed by laminating and bonding a
plurality of metals are used in various fields. In particular, materials
having a Ti (titanium) surface layer are being developed for use in fuel
cell bipolar plates exposed to harsh corrosive environments. Examples of
prior patent documents which disclose a fuel cell bipolar plate
fabricated from a material having a Ti surface layer includes, e.g.,
JP-A-2006-210320 and JP-A-2005-158441.

[0006]The Ti is a typical corrosion resistant metal. However, corrosion
resistant metals are very expensive and therefore cannot be used freely
for bipolar plates of consumer fuel cells. To address this problem, the
present inventors are developing bipolar plates fabricated from a metal
composite made of an inexpensive core material cladded with a corrosion
resistant metal on both surfaces (e.g., JP-A-2006-210320).

[0007]Generally, in fuel cells, the fuel gas and oxidant gas (e.g., air)
used for generating electricity are humidified (e.g., concentrated
methanol is humidified (diluted) with water generated at the air
electrode) in order to improve the power generation characteristics.
These gases are supplied to each MEA (Membrane Electrode Assembly) via
through-holes formed in the bipolar plates. In other words, each bipolar
plate is required to be provided with through-holes, which are part of
the fuel and oxidant gas conduits.

[0008]A problem with conventional metal composites for fuel cells is that
the hole wall of the core layer region of the through-hole is exposed to
the ambient environment, and therefore dissolution (corrosion) of the
core material can occur due to dew condensation or a pH change caused by
impurity ions contained in the oxidant gas. Furthermore, when an Al
(aluminum) is employed as the core metal in order to reduce the weight of
a bipolar plate, more reliable anti-corrosion protection is needed
because the Al is far less resistant to such core dissolution than a
stainless steel.

[0009]A method for covering exposed core surfaces is disclosed, for
example, in the above-mentioned JP-A-2005-158441 in which, in order to
suppress corrosion of the core metal of the bipolar plate, the
through-hole wall and plate surfaces surrounding the hole are adhesively
covered by a film coating. However, the method according to the above
JP-A-2005-158441 covers each through-hole individually, and therefore can
cause an increase in the number of components and the manufacturing cost.
Further, in this method, the resin film can block the through-hole, or
can thicken the portion of the bipolar plate surrounding each
through-hole, thus possibly incurring assembly difficulty.

SUMMARY OF THE INVENTION

[0010]Under these circumstances, it is an objective of the present
invention is to provide a metal composite for fuel cells and a fuel cell
bipolar plate fabricated from the metal composite, which can prevent the
core material from being directly exposed to the corrosive environment
while suppressing an increase in the number of components. Furthermore,
it is another objective of the present invention to provide a fabrication
method for the metal composite providing fabrication simplicity and a low
cost.

[0011](1) According to one aspect of the present invention, there is
provided a metal composite for fuel cells, which includes: a core of a
metal; cladded layers of a corrosion resistant metal covering both
surfaces of the core; and a through-hole formed through the core and
cladded layers, which has, on a hole wall of the core region of the
through-hole, a concave portion which is recessed relative to hole walls
of the cladded layer regions of the through-hole.

[0012]In the above aspect (1) of the present invention, the following
modifications and changes can be made.

[0013](i) Projected hole wall portion of each cladded layer region of the
through-hole is bent inwardly.

[0014](ii) The concave portion is filled with a dissolution-inhibiting
material, such as a resin, for preventing dissolution of the core.

[0015](2) According to another aspect of the present invention, there is
provided a fuel cell bipolar plate fabricated from the metal composite of
the above aspect (1) of the present invention.

[0016](3) According to still another aspect of the present invention,
there is provided a fabrication method for a metal composite for fuel
cells, which includes the steps of: covering both surfaces of a core of a
metal with a cladded layer of a corrosion resistant metal; forming a
through-hole through the core and cladded layers; and etching a hole wall
of the core region of the through-hole to form a concave portion which is
recessed relative to hole walls of the cladded layer regions of the
through-hole.

[0017]In the above aspect (3) of the present invention, the following
modifications and changes can be made.

[0018](iii) After the step of formation of the concave portion, there is
an added step of forcing a punch against and inwardly bending projected
hole wall portion of each cladded layer region of the through-hole which
is projected relative to the hole wall of the core region of the
through-hole.

[0019](iv) There are added steps of: anodizing a surface of the concave
portion; and applying a boehmite treatment to the anodized surface.

[0020](v) There is an added step of filling a dissolution-inhibiting
material to the concave portion for preventing dissolution of the core.

ADVANTAGES OF THE INVENTION

[0021]The metal composite according to the invention provides extended
service lives of fuel cell components such as bipolar plates and greater
flexibility of choice of the core material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0022]FIG. 1(a) is a schematic illustration showing a plan view of a metal
composite for fuel cells according to a first preferred embodiment of the
present invention; and

[0023]FIG. 1(b) is a schematic illustration showing a cross-sectional view
along 1B-1B line in FIG. 1(a).

[0024]FIGS. 2(a) and 2(b) are schematic illustrations showing a
cross-sectional view of an exemplary fabricating method for the metal
composite shown in FIG. 1.

[0025]FIG. 3 is a photograph showing a core and cladded layers after the
core has been etched to form a concave portion.

[0026]FIG. 4 is a schematic illustration showing a cross-sectional view of
a principal portion of a modification of the FIG. 1 metal composite.

[0027]FIG. 5 is a photograph showing a cladded layers and
dissolution-inhibiting material after a resin has been filled into the
concave portion at the core in FIG. 3.

[0028]FIG. 6 is a schematic illustration showing a perspective view of a
bipolar plate for a polymer electrolyte fuel cell fabricated from the
metal composite in FIG. 1 and a stack structure of the polymer
electrolyte fuel cell.

[0029]FIG. 7(a) is a schematic illustration showing a cross-sectional view
of an exemplary fabricating method of a metal composite for fuel cells
according to a second embodiment of the present invention; and FIG. 7(b)
is a schematic illustration showing a cross-sectional view of a principal
portion of the metal composite according to the second embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0030]Preferred embodiments of the present invention will be described
below with reference to the accompanying drawings. However, the present
invention is not limited to the embodiments described herein.

First Embodiment of the Invention

[0031](Structure of Metal Composite)

[0032]FIG. 1(a) is a schematic illustration showing a plan view of a metal
composite for fuel cells according to a first preferred embodiment of the
present invention; and FIG. 1(b) is a schematic illustration showing a
cross-sectional view along 1B-1B line in FIG. 1(a).

[0033]As shown in FIGS. 1(a) and 1(b), a metal composite 1 for fuel cells
according to the first embodiment includes: a core 2 of a metal plate;
cladded layers (corrosion resistant cladded layers) 3 covering both
surfaces of the core 2; and through-holes 4 penetrating both the core 2
and cladded layers 3 in the thickness direction (perpendicular to the
drawing plane of FIG. 1(a) and vertical direction as viewed in FIG.
1(b)). In addition, on a hole wall of the core 2 region of each
through-hole 4 of the metal composite 1, there is formed a
circumferentially-extending concave portion 5 that is recessed relative
to a hole wall of the cladded layer 3 regions on both sides of the core
2. In other words, in the cladded layer 3 regions of each through-hole 4
on both sides of the core 2 region, there is formed a
circumferentially-extending and centrally-projecting projection portion
3p.

[0034]Further, each concave portion 5 is filled with a
dissolution-inhibiting material 6 such as a resin in order to prevent
dissolution of the core 2 into the ambient environment in use, operation
and storage of the metal composite 1. The dissolution-inhibiting material
6 is preferably filled into each concave portion 5 in such a manner as to
prevent the hole wall of the core 2 region from being exposed to the
interior of the through-hole 4. FIG. 1(b) illustrates an example in which
the dissolution-inhibiting material 6 is filled into the concave portion
5 in such a manner as to have a slightly concaved surface extending from
both projection portions 3p.

[0035]Besides, FIG. 1 illustrates an example in which four through-holes 4
with a circular cross section are formed near the four corners of the
metal composite body 1b. In addition, on a central portion of both faces
of the metal composite body 1b, a ridge-and-groove structure 1r serving
as a gas conduit is formed by, for example, folding or pressing.

[0036]The core 2 is made of a metal such as Al, Cu (copper), Fe (iron), Ni
(nickel), and Pb (lead) or an alloy thereof, or stainless steel, or a
stainless-steel/metal-composite. The cladded layers 3 are made of a
corrosion resistant metal such as Au (gold), Pd (palladium), Pt
(platinum), Ni, Ta (tantalum), Nb (niobium), W (tungsten), Ti, or an
alloy thereof, or stainless steel.

[0037]Although an Al or an Al alloy is preferable for the core 2 from the
weight and price points of view, other common metals such as stainless
steel, Ni, and Cu may also be used. On the other hand, the metal used for
the core 2 is preferably dissolvable relatively than the cladded layers 3
in an etchant such as an acid solution and an alkaline solution (both of
which will be described later). In other words, any metal may be used as
long as the etch rate of the core 2 is faster than that of the cladded
layer 3. This causes the hole walls of the cladded layers 3 to project
relative to the hole wall of the core 2, thus preventing or suppressing
exposure of the core 2 to the ambient environment.

[0038]For the cladded layers 3, besides Ti and Ti alloys, any metal can be
used such as stainless steel alloys, and Ta, Nb, W, Ni, alloys thereof,
and compounds thereof as long as the corrosion resistance is excellent
and the adverse effects on the fuel cell characteristics are tolerable.

[0039](Fabrication Method for Metal Composite)

[0040]Next, an exemplary method of fabricating the metal composite 1 will
be described. This embodiment will be described by way of an example in
which Al or an Al alloy is employed for the core 2 in order to reduce the
weight, and Ti or a Ti alloy is employed for the cladded layers 3 in
order to improve the corrosion and heat resistance.

[0041]Specifically, a 0.2-mm-thick Al alloy (5000 series in Al--Mg system)
is used as the core 2 and a 0.03-mm-thick Ti is used as the cladded
layers 3. Firstly, these are laminated together to prepare a thin clad
material, which is then press molded to form the metal composite body 1b
(prior to formation of the later-described concave portion 5). Here, the
through-holes 4 may be formed simultaneously with or after the press
molding.

[0042]FIGS. 2(a) and 2(b) are schematic illustrations showing a
cross-sectional view of an exemplary fabricating method for the metal
composite shown in FIG. 1. As shown in FIG. 2(a), the through-holes 4 are
formed through the metal composite body 1b composed of the core 2 and
cladded layers 3. Then, the hole wall of the core 2 region exposed to the
interior of each through-hole 4 is chemically etched to form the concave
portion 5 that is recessed relative to the hole walls of the cladded
layer 3 regions of the through-hole 4. As described above, when an Al
alloy is used for the core 2, it can be chemically etched using an
etchant such as an acid solution and an alkaline solution.

[0043]Specifically, the metal composite body 1b as shown in FIG. 2(a) is
immersed, e.g., in a 1N aqueous sodium hydroxide solution for 5 minutes.
This causes only the core material to dissolve in the aqueous sodium
hydroxide solution and causes the hole wall of the core 2 region to be
recessed relative to the hole walls of the cladded layer 3 regions on
both sides. In this manner, the concave portion 5 such as shown in FIG.
2(b) can be formed.

[0044]FIG. 3 is a photograph showing the core 2 and clad-layers 3 after
the core 2 has been etched to form the concave portion 5. However, for
clarity's sake, FIG. 3 shows, instead of the interior of the through-hole
4, an outer corner of the metal composite body 1b. Then, the
dissolution-inhibiting material 6 is filled in each concave portion 5,
thereby obtaining the metal composite 1 such as shown in FIGS. 1(a) and
1(b). As the dissolution-inhibiting material 6, there can be used, e.g.,
a viscosity-modified ARALDITE (Registered Trade Mark, a two-component
adhesive containing an epoxy resin as the main component and a
polyamidoamine as a hardener). Furthermore, after the formation of the
concave portion 5 and prior to the filling of the dissolution-inhibiting
material 6, the concave portion 5 may be subjected to an anodization
followed by a boehmite treatment (i.e., filling of a boehmite [alumina
monohydrate or aluminum oxide hydroxide] as a resin filler [inorganic
filler]).

Effects and Advantages of Preferred Embodiments

[0045]Effects and advantages of the first embodiment will be described.

[0046]In the metal composite 1 according to the first embodiment, each
through-hole 4 has, on the hole wall of the core 2 region, the concave
portion 5 that is recessed relative to the hole walls of both cladded
layer 3 regions. When the core metal is dissolved as an ion (e.g.,
Al3+) from the hole wall of the core 2 region in the concave portion
5, the ion is difficult to diffuse out of (outflow from) the concave
portion 5. Then, the ion is easy to be oxidized and precipitated so that
an oxide layer covers the hole wall of the core 2 region. Therefore,
dissolution of the core 2 into the ambient environment is suppressed
compared to conventional arts in which the hole walls of the core region
and cladded layer regions are coplanar. In addition, the concave portion
5 of each through-hole 4, which is formed, for example, by slightly
chemically etching the hole wall of the core region, is filled with the
dissolution-inhibiting material 6, thus preventing the hole wall of the
core 2 region from being exposed to the interior of the through-hole 4.

[0047]That is, the hole wall of the core 2 region of each through-hole 4
is not directly exposed to the corrosive environment. So, a low corrosive
resistant metal such as an Al and Al alloy can be used for the core 2,
because dissolution of such metal is prevented even when there occurs dew
condensation or a pH change due to impurity ions contained in the oxidant
gas. Thus, the service lives of various fuel cell components (such as
bipolar plates) fabricated of the metal composite 1 can be extended.

[0048]Since dissolution of the core 2 is suppressed, as the core 2
material there can be used such metals that can cause, if dissolved from
the core 2, damaging effects on the operation of the fuel cell. This
provides greater flexibility in the choice of the core material.

[0049]And, the cladded layers 3 are of a corrosion resistant metal and
therefore are less readily etched than the core 2. Thus, the core 2 can
be selectively etched. Furthermore, the concave portion 5 of the core
metal region is formed so as to have a recessed surface, thereby allowing
efficient filling of the dissolution-inhibiting material 6 into the
concave portion 5.

[0050]In this embodiment metal composite 1, the dissolution-inhibiting
material 6 is filled into the concave portion 5 and is spread over the
entire surface thereof due to surface tension of a resin; therefore,
uncovered (exposed) hole wall areas of the core 2 region can be
significantly reduced compared to conventional arts in which a resin is
applied on the inner wall of such a through-hole without such concave
portion. Thus, the hole wall of the core 2 region of each through-hole 4
can be protected more stably and reproduced.

[0051]With the fabrication method according to the present invention, the
concave portion 5 can be readily formed just by chemically etching the
hole wall of each through-hole of the metal composite body 1b. Therefore,
the metal composite 1 capable of preventing dissolution of the core 2 can
be readily fabricated from a relatively small number of parts, thus
leading to low fabrication cost.

[0052]In addition, the surface of the concave portion 5 formed on the core
2 region is anodized to form an oxide film, which is then subjected to a
boehmite treatment. This enhances the adhesiveness of the
dissolution-inhibiting material 6 to the oxide film, thereby more
assuredly preventing dissolution of the core 2.

[0053]FIG. 4 is a schematic illustration showing a cross-sectional view of
a principal portion of a modification of the FIG. 1 metal composite. As a
modification of the FIG. 1 metal composite 1, the dissolution-inhibiting
material 6 may be filled in the concave portion 5 in such a manner as to
have a slightly convex surface extending from both projection portions 3p
as shown in the metal composite 41 of FIG. 4. FIG. 5 is a photograph
showing the cladded layers 3 and dissolution-inhibiting material 6 after
a resin has been filled into the concave portion 5 at the core 2 in FIG.
3. Similarly to FIG. 3, FIG. 5 shows, instead of the interior of the
through-hole, an outer corner of the metal composite body 1b.

[0054](Bipolar Plate for Fuel Cell)

[0055]Next, an exemplary application of the metal composite 1 will be
described with reference to FIG. 6.

[0056]FIG. 6 is a schematic illustration showing a perspective view of a
bipolar plate for a polymer electrolyte fuel cell fabricated from the
metal composite in FIG. 1 and a stack structure of the polymer
electrolyte fuel cell. As shown in FIG. 6, the metal composite 1 can be
used for the bipolar plate of a fuel cell stack 31. Unlike the
square-shaped metal composite 1 in FIG. 1(a), in the FIG. 6 example, the
metal composite 1 is rectangular in shape and the through-hole is
substantially oval in cross section.

[0057]The fuel cell stack 31 is a polymer electrolyte fuel cell (PEFC) and
is configured by stacking multiple-unit cells C. This fuel cell stack 31
can be also used for direct methanol fuel cells (DMFCs), which are
mounted as a portable power source in portable equipment such as cellular
phones and notebook PCs.

[0058]The unit cell C includes: a first metal composite 1 serving as a
bipolar plate (also called "separator") for separating the fuel and
oxidant gas conduits; a first sealing gasket 32 having a central opening;
an electricity generating assembly 33; a second sealing gasket 32; and a
second metal composite 1. The ridge-and-groove structure 1r on one side
of the metal composite 1 serves as a conduit for carrying a fuel gas
(e.g., hydrogen obtained from methanol), and that on the other side
serves as a conduit for carrying an oxidant gas (e.g., air).

[0059]The electricity generating assembly 33 includes: an MEA (a polymer
electrolyte membrane supporting catalyst layers) 33m; and gas diffusion
layers 34 provided on both surfaces of the MEA 33m and serving as
electrodes. One of the gas diffusion layers 34 serves as the fuel
electrode, and the other the air electrode.

[0060]The bipolar plates account for major part of the cost of such a fuel
cell stack 31. So, it is very effective in reducing the cost of a fuel
cell that the low-cost and readily-manufacturable metal composite 1
according to the present invention can be provided for such a fuel cell
bipolar plate. In addition, the metal composite 1 can also be utilized
for the gasket 32 and the frame of the electricity generating assembly
33.

Second Embodiment of the Invention

[0061]A second embodiment of the present invention will be described. FIG.
7(a) is a schematic illustration showing a cross-sectional view of an
exemplary fabricating method of a metal composite for fuel cells
according to a second embodiment of the present invention; and FIG. 7(b)
is a schematic illustration showing a cross-sectional view of a principal
portion of the metal composite according to the second embodiment.

[0062]As shown in FIG. 7(b), in a metal composite 51 for fuel cells
according to the second embodiment, a concave portion 5 is formed in each
through-hole 4 similarly to the first embodiment shown in FIG. 2(b).
Unlike the first embodiment, the projected hole wall portion
(corresponding to the projection portion 3p in FIG. 2(b)) of the cladded
layer regions of each through-hole 4 is bent inwardly to form a bent
projection 3f. And, a dissolution-inhibiting material 6 is filled into
the space of the concave portion 5 sandwiched between both bent
projections 3f.

[0063]In a method of fabricating the metal composite 51 according to the
second embodiment, firstly, the concave portion 5 is formed in each
through-hole 4. Then, a punch having a head diameter slightly smaller
than that of the through-hole 4 is pressed from both sides so that the
projected hole wall portion (corresponding to the projection portion 3p
in FIG. 2(b)) of each cladded layer 3 is bent inwardly to form the bent
projection 3f.

[0064]Although the bent projections 3f (concave portion 5) alone provide a
sufficient effect on reducing the corrosion of the core metal because of
the same mechanism in the first embodiment, the filling of the
dissolution-inhibiting material 6 in the space of the concave portion 5
sandwiched between both bent projections 3f can offer a more beneficial
effect. After the filling of the dissolution-inhibiting material 6, the
metal composite 51 shown in FIG. 7(b) is obtained.

[0065]The metal composite 51 has the same effects and advantages as the
metal composite 1 of the first embodiment. In addition to these
advantages, in this embodiment, after the etching of the core 2, the
projected hole wall portion (corresponding to the projection portion 3p
in FIG. 2(b)) of each cladded layer 3 is bent inwardly to form the bent
projection 3f, thereby further stabilizing the filling of the
dissolution-inhibiting material 6.

[0066]While the concave portion 5 is formed by chemical etching in the
above embodiments, electrochemical etching such as electrolytic etching
may be used. In this case, the etching is performed by immersing the
composite metal body 1b (used as the anode electrode) in an electrolyte
and applying a positive voltage thereto. In addition, the electrolytic
etching may be continuously followed by anodization of the surface of the
concave portion 5 of the core.

[0067]The present invention can also be applied to a metal composite in
which another conductive coating is applied over each cladded layer 3. In
this case, when an organic conductive coating that can react with or be
dissolved in an etchant is used, the coating is preferably applied after
the etching process for forming the concave portion 5.

[0068]Although the invention has been described with respect to the
specific embodiments for complete and clear disclosure, the appended
claims are not to be thus limited but are to be construed as embodying
all modifications and alternative constructions that may occur to one
skilled in the art which fairly fall within the basic teaching herein set
forth.